JP4-236846A discloses a control device for determining the occurrence of an ON-failure in which a lock-up clutch cannot be released if the rotation speed of an input shaft and that of an output shaft of a torque converter continue to be equal although a release instruction is output to the lockup clutch.
In recent years, vehicles in which a lock-up clutch is released only when the vehicle starts and is engaged in other times have been on the increase to improve fuel economy. In other words, vehicles in which most operating regions the lock-up clutch is engaged and only at very low vehicle speed is the lock-up clutch released have been on the increase. Thus, such vehicles have a problem that there is less opportunity to instruct the release of the lock-up clutch and determine the ON-failure of the lock-up clutch and it is difficult to discover the ON-failure early.
As an alternative to this, it is possible to increase the chance of determining the ON-failure by outputting a release instruction to the lock-up clutch during normal traveling, or by narrowing the lock-up region and widening the converter region, for example, to discover the ON-failure early. However, this presents a problem of deteriorating fuel economy since a transmission loss in the torque converter becomes large.